1. Field of the Invention
The present invention relates to a method for generating an image of an examination subject with a tomography-capable X-ray device, particularly with An X-ray computed tomography device, having a multi-row X-ray detector array, an X-ray radiator that rotates around a system axis and emits a conical X-ray beam, and a positioning device by allowing an examination subject to be positioned in a direction parallel to the system axis at various z-positions relative to the X-ray radiator.
2. Description of the Prior Art
In two-dimensional computed tomography, raw data are acquired for subsequent image reconstruction by means of fan beam devices, for example. A fan beam device of this type has a single detector row with individual detector elements disposed in the azimuthal direction. In conformance with this detector geometry, a planar X-ray fan is generated by means of a gating (diaphragm) device. While the X-ray radiator rotates, a number of different projections of the examination subject, namely the patient, are acquired. If the relative distance between the X-ray radiator and the examination subject in a direction parallel to the system axis remains unchanged during this rotation, a single slice, i.e., a two-dimensional portion, of the examination subject is scanned. The result of a subsequent image reconstruction employing algorithms known as convolution algorithms (filtered back projection) is then a two-dimensional topogram or CT image of the scanned slice perpendicular to the rotational axis or system axis.
For the purpose of scanning a volume of the examination subject, raw data of a respective slice are generated in succession at different relative positions (z-positions) of the X-ray radiator relative to the examination subject along a direction parallel to the system axis in a sequence scan, and for each slice a two-dimensional image reconstruction is performed. The tomograms that result from the individual image reconstructions can then be assembled into a 3D image in a stacked fashion.
Overview exposures or topograms are a known means of locating a desired slice or a desired volume in a subject or patient that is to be scanned in the z-direction. For this projection technique, the scanning system remains in a fixed angle position, e.g. X-ray tube and detector above and below the patient. The patient is then moved through the measuring opening. The resulting row attenuation profiles are assembled into a shadow image in the computer and displayed on an image monitor. The desired imaging or scan region can then be selected using marks that can be mixed in, and the positioning of the system components necessary to scan this region can automatically occur. Suitable methods and devices for this are known from German PS 42 23 430 and German PS 197 21 535, for example.
Spiral scanning, wherein the X-ray radiator travels along a helical path around the subject with continuous motion along the system axis, was developed specifically for improving the image contrast. Spiral scanning also can be performed by means of the above-mentioned 2D reconstruction technique by the initial determination of planar datasets (using procedures known as spiral algorithms or slice interpolation processes) from the data that arise during the spiral scan, in a preliminary step prior to the actual image reconstruction.
Computed tomography devices with multi-row X-ray detector arrays have been recently developed. The advantages of these devices are better image contrast, smaller radiation dose for the patient, and shorter examination time, as well as a reduction of movement artifacts associated with movements of the patient during examination (e.g. heart exam). The gating of the X-ray beam onto such a multi-row X-ray detector is no longer two-dimensional as in a fan beam device, but instead is three-dimensional, hence the term conical beam devices (Cone Beam CT Scanner). Due to the cone-shaped scan, a correction of the oblique beam path in the volume is generally required. This requires special 3D reconstruction methods, known as cone beam image reconstruction methods. A distinction is made between approximative methods and exact methods.
Approximative algorithms, for instance algorithms based on a 2D Radon inversion, are described in the article “Advanced Single-Slice Rebinning in Cone-Beam Spiral CT” (M. Kachelriess, S. Schaller, W. A. Kalender; Med Phys. Vol. 27, 4 (2000): 745-772) and in the article “Novel Approximate Approach For High-Quality Image Reconstruction In Helical Cone Beam CT At Arbitrary Pitch” (S. Schaller, K. Stierstorfer, H. Bruder, M. Kachelriess, T. Flohr, SPIE Med. Imag. Conf., V. 4322 (2001): 113-127. These algorithms are highly flexible, for instance with respect to the free adjustability of the pitch (ratio of z shift per rotation to slice thickness); however, they are not sufficiently precise in detector arrays having a larger number of rows, for instance more than four rows, because the error emerging from the approximation grows as the cone angle increases.
Therefore, methods also have been developed which precisely account for the cone angle. The article “Exact Radon Rebinning Algorithm For The Long Object Problem In Helical Cone-Beam CT” (S. Schaller, F. Noo, F. Sauer, K. C. Tam, G. Lauritsch, T. Flohr; Proc. of the 1999 Int. Meeting on Fully 3D Image Reconstruction (1999): 11-14) and the article Cone-Beam Filtered-Backprojection Algorithm For Truncated Helical Data (H. Kudo, F. Noo, M. Defrise; Phys. Med. Biol., v. 43 (1998): 2885-2909) describe such methods for flat detectors with a large number of rows, e.g. 256 rows, and with a large cone spread. These exact cone beam algorithms, however, require a maximal table displacement of approximately 1.5 times the detector height for optimum use of the detector data and the applied dose. Such high displacement speed is undesirable in many instances.
In order to determine a complete dataset that is sufficient for a 3D reconstruction, a criterion known as Tuy's condition must be satisfied, as describe& in “An Inversion Formula For Cone Beam Reconstruction” (H. Tuy, SIAM Journal on Applied Mathematics, v. 43, Nr. 3 (1983): 546-552. According to this condition, each plane through the image subject must be intersected by the path of the X-ray focus at least once. The dataset that is generated in a rotation scan such as a sequence scan alone is therefore insufficient for a 3D reconstruction. In other words, the scan in the 3D Radon space is incomplete. Ideally, this should contain all planar integrals of the beam cone of planes oriented randomly in the examination subject, as is mathematically expressed by Equation (15) of the Tuy article.
In order to obtain a complete dataset (data record) for a cone beam image reconstruction process, according to U.S. Pat. Nos. 6,014,419 and 5,170,439, the rotation scan is combined with a linear scan, and the total data volume is used as the starting dataset for the image reconstruction. According to the cited references, a combination of a linear scan and a rotational scan takes place multiple times in succession until the relevant volume has been scanned completely. In this “circle and line orbit,” the rotation of the X-ray radiator must be interrupted after each step of the rotation scan in order to be able to execute one of the many linear scan steps while the radiator is not rotating. As described in U.S. Pat. No. 6,014,419, this constant interrupting of the rotational movement is undesirable and creates a time disadvantage. This reference therefore proposes an alternative, known as a “circle and helix scan,” which is intended to reduce the overall data acquisition time substantially.